Psychrometric Calculator and Chart

How it works

Moist air is dry air plus water vapor. Pick the dry-bulb temperature and one other property; the pressure fixes the rest. The page uses the equations in the ASHRAE Handbook of Fundamentals.

W = 0.621945 pw ÷ (P − pw)

h = 1.006 T + W (2501 + 1.86 T)

The saturation pressure pws comes from the Hyland–Wexler equation, relative humidity is pw ÷ pws, and the dew point is the temperature at which pws equals pw. The wet-bulb temperature is found by solving the adiabatic-saturation energy balance. The chart always plots in °C and g/kg for a pressure equal to the one you chose. For heating and cooling energy use enthalpy differences: Q = ṁ × Δh.

Worked example

Air at 25 °C and 50 % relative humidity at sea level. Saturation pressure at 25 °C is 3 169 Pa, so pw = 0.5 × 3 169 = 1 585 Pa. W = 0.621945 × 1 585 ÷ (101 325 − 1 585) = 9.88 g/kg. The dew point is 13.86 °C, the wet bulb is 17.89 °C, the enthalpy is 50.32 kJ/kg and the specific volume is 0.8580 m³/kg.

Things to know

Pressure matters: at 1 500 m the air holds more water for the same relative humidity and the specific volume is about 20 % larger. Use the altitude option or enter the actual station pressure (not the sea-level-corrected weather report pressure).

Below 0 °C the saturation pressure is taken over ice, so the dew point below freezing is really a frost point.

Wet-bulb readings from a sling psychrometer depend on air speed (about 3 m/s or more); a still wick reads high.

Above about 60 °C the simple enthalpy equation is still fine, but fogging, condensation and non-ideal gas effects grow. This is not a steam table.

Condensation on a surface starts when its temperature is below the dew point of the surrounding air.

Common questions

What is the difference between dew point and wet bulb?

The dew point is the temperature at which the air would become saturated if cooled at constant moisture content. The wet bulb is the temperature of a wetted thermometer in moving air and lies between the dew point and the dry bulb.

Why does enthalpy include moisture?

Water vapor carries latent heat of about 2 501 kJ/kg. Drying air removes much more energy than cooling it by a few degrees, which is why dehumidification dominates air-conditioning load in humid climates.

How accurate is the result?

The equations reproduce ASHRAE table values to better than 0.1 % for typical building and process conditions at pressures from about 50 to 150 kPa.

Sources

• ASHRAE Handbook – Fundamentals, Chapter 1 (Psychrometrics): equations 5, 6, 12, 22, 32, 35, 36.
• Hyland, R. W. and Wexler, A., Formulations for the Thermodynamic Properties of the Saturated Phases of H2O, ASHRAE Transactions 89(2A), 1983.

Last reviewed: November 2, 2026